Conventional berthing methods and systems used in most port terminals usually involve the use of a dock, or wharf, fixed to the bottom and provided with fendering means designed to absorb the environmental loads to which the dock is subjected as a result of wind forces, currents and waves. For large vessels, like the ones that transport petroleum products and other specialized products such as liquefied natural gas and compressed natural gas, berthing dolphins are often placed adjacent to or separately from the dock in order to provide additional stability to the ship. Berthing dolphins are freestanding marine structures embedded into the bottom and provided with fendering means for absorbing environmental loads during berthing. Hawsers and similar types of cables and ropes are passed through hawseholes located in the bow of the ship and fastened so as to moor the ship to the dock and hold the ship in proper relationship with the terminal. In these “hard berthing” techniques, the vessel being berthed is made to come into and remain in contact with the fixed marine structures, i.e., the dock terminal and the berthing dolphins.
While these hard berthing techniques may be adequate for loading and unloading cargo to and from wharfs and docks in protected waters, they are not adequate for the loading and unloading of cargoes in unprotected waters during much of the environmental conditions that prevail in such locations. Loading or unloading of cargoes in unprotected waters often requires that the berthing operation be conducted quickly, with minimum separation distance between the vessel and the offshore platform and with high degree of accuracy. Otherwise, the transfer operation runs the risks of causing accidental cargo spills and frozen lines, with their attendant safety and environmental hazards, not to mention expensive product losses.
The use of single-point-mooring buoys is another example of conventional berthing methods and systems. Single-point-mooring buoys, however, are used only at the bow of the vessels, not alongside the vessels, and their use requires that specialized fittings and manifolds be installed on the vessels in order to load and unload the vessels.
It is apparent that a need exists for a technique whereby an offshore structure in unprotected waters, such as a deepwater terminal or platform, is able to receive cargo from large vessels in a safe and efficient manner without substantial periods of shut down due to normal environmental conditions. The present invention is directed toward providing such a technique.
It is an object of the present invention to provide a method and a system for the proper and safe berthing of ships in unprotected waters during most environmental conditions. It is also an object of the present invention to provide a method and a system for berthing marine vessels under ambient conditions that require high degrees of safety, efficiency and effectiveness. Another object of this invention is to provide a commercially practicable method for the berthing of a difficult-to-handle cargo ship alongside in unprotected waters, which method reduces the effects of environmental loads between the ship and the berth while utilizing conventional components in a novel fashion. A specific object of the invention is to provide an environmentally attractive method for berthing large cryogenic-fluid-transport tankers in spatial relationship to offshore platforms and similar structures that serve as deepwater port terminals. A further object of the present invention is to provide a technique that increases the time-available window for berthing a tanker at a fixed structure in open waters in a safe and cost-effective manner. Still a further object of the present invention is to provide a system for berthing vessels alongside in open waters, which system is able to accommodate conventional vessels for loading and unloading and does not require the vessels to have special bow or stern loading manifolds or fittings. These and other objects of the invention will be apparent to those skilled in the art from the description that follows.